Molecular mechanics calculations (MM3) on alkyl iodides

Anharmonic Molecular Mechanics: Ab Initio Based Morse Parametrizations for the Popular MM3 Force Field

Methodologies for creating reactive potential energy surfaces from molecular mechanics force-fields are becoming increasingly popular. To date, molecular mechanics force-fields in biochemistry and small molecule organic chemistry tend to use harmonic expressions to treat bonding stretches, which is a poor approximation in reactive and nonequilibirum molecular dynamics simulations since bonds are often displaced significantly from their equilibrium positions. For such applications there is need for a better treatment of anharmonicity. In this contribution, Morse bonding potentials have been extensively parametrized for the atom types in the MM3 force field of Allinger and co-workers using high level CCSD(T)(F12*) energies. To our knowledge this is among the first instances of a comprehensive parametrization of Morse potentials in a popular organic chemistry force field. In the context of molecular dynamics simulations, these data will: (1) facilitate the fitting of reactive potential energy surfaces using empirical valence bond approaches and (2) enable more accurate treatments of energy transfer.

Potential energy functions

When energy is a critical quantity, accurate biomolecular simulations rest in substantial part on accurate potential energy functions (force fields). Improvements in methodology for determining parameters--particularly, in the systematic use of computational data obtained from quantum chemical calculations--and enhancements in functional form are leading to better potential energy functions. New calculations have been developed for water (including calculations that incorporate electronic polarizability to take account of the degree to which a molecule can be polarized), proteins, nucleic acids, carbohydrates, lipids, and general organic molecules. Most notably, two new biomolecular force fields have recently been derived and significant redeterminations of the parameters of two existing biomolecular force fields have been carried out. Some progress has also been made in incorporating polarizability into potential energy functions for molecules in general and in improving the treatment of metal-ligand interactions in systems of biomolecular interest.